Chromate treatments have been the backbone for some years in the control of corrosion, deposition, scaling and general fouling in industrial cooling water systems. Environmental concern for waste-water quality has given rise to legislative standards requiring low levels of chromate in discharged waste water. Some proposed standards favor zero chromate discharge. Obviously, the alternatives to industries using chromates are either to discontinue use and find substitutes, or to reduce or remove chromates from the effluent waters. This invention is concerned with the latter alternative, and in developing a chromate reduction process which will minimize cost to industry and be relatively easy to maintain in use.
Prior art methods of chromate reduction generally involve reduction of the hexavalent chromate ion to the trivalent chromium ion followed by elevation of pH for precipitation of the reduced chromium, or anion exchange removal. Reducing agents employed most commonly for this purpose include sulfur dioxide, ferrous sulfate, sodium bisulfate sodium metabisulfate, sodium sulfite and hydrogen sulfide. The reduced chromium precipitates out as chromium hydroxide upon elevation of pH, if precipitation is used for removal.
The basic problem with use of the above reducing agents is the lack of selectivity in the reduction step. Methods using ferrous or other oxidizable metal ions or sulfur dioxide require high chemical feed rates since the entire cooling water blowdown must be treated to reduce the small quantity of chromate present. Also, use of iron results in the production of ferric hydroxide upon neutralization, which is extremely difficult to remove from water. Use of sulfur dioxide is costly since the amount employed must be sufficient to reduce the chromate and any other reducible substances in the water, including dissolved oxygen. The anion exchange method of removing chromate puts chromate in competition with other divalent or trivalent anions present and selectivity is low, resulting in high chromium leakage and incomplete resin utilization. The precipitation method of removal requires higher costs due to the necessity of a sedimentation tank of sophisticated design to handle the chromium hydroxide sludge, followed by transfer to a lagoon which must be large enough to handle flows of 25 to 50 gallons per minute or higher from the tanks. However, where enough land is available, this method is perfectly acceptable.